This invention relates to telecommunication networks which contain switching and signaling network elements, and more particularly telephonic switch network elements and signal transfer points having Signaling System 7 controllers for the routing of messages.
The Signaling System No. 7 protocol has been mandated for out-of-band signaling communication involving telecommunication network elements and has been implemented worldwide. Signaling System No. 7 (SS7) network elements are each uniquely identified by a Message Transfer Part (MTP) Signaling Point Code (PC). Network administrators set up routes through a SS7 network so that the network elements can send Message Signal Units (MSUs) from any network element to any other network element in the telecommunication network.
Two types of network elements are public switched telephone network (PSTN) switches and signal transfer points (STPs). PSTN switches control the telephone and service traffic that is provided by the telecommunications operating company. A key to the success of a telecommunication service provider is for its PSTN switches to provide continuous service. It is critical that the customers of the service provider have the ability to communicate through the telecommunications network via a PSTN switch associated with the particular customer. Moreover, there are federal (United States) and world (International Telecommunications Unionxe2x80x94Telecommunications (ITU-T)) reporting requirements for SS7 failures (xe2x80x9coutagesxe2x80x9d) above a certain threshold. The STPs act as signaling hubs for concentrating SS7 signaling link sets so the PSTN switches do not need to be fully interconnected. However, the concentration of signal link sets from the PSTN switches to a central PSTN switch by the STP results in an artificial limit on the scaleable capacity of the PSTN switches connected to the STP.
Network elements, such as the PSTN switches and the STPs, are interconnected by a group of signaling links called a link set that all originate and terminate between the same pair of Signaling Point Codes. A benefit of Signaling System No. 7 (SS7) is that the signaling is out-of-band. The out-of-band signaling enables the routing of the telephonic signals for setting up and ending calls separate from the communication or bearer channels. The bearer channels between switches are grouped together into trunk groups. A signaling route for out-of-band signaling is assigned to transport the signaling for each trunk group. The signaling links between the STP and the PSTN switch are referred to as A-Links and are identified by three parameters, the two Point Codes (PCs) at each end of the link and a Signaling Link Code (SLC). When multiple links are deployed between PCs the signaling links are combined together. The grouping of signaling links associated with a trunk group is commonly referred to as a link set. Associated common-channel signaling links between two PSTN switches that are fully associated (transport only signaling messages for associated trunk groups) are referred to as F-links and are identified by the same three parameters as an A-Link.
Referring to FIG. 1, an SS7 network diagram illustrating link set deployment with multiple PSTN switches 14, 16, 18, a central PSTN switch 20, trunk groups 30, 32, 34, and a pair of redundant STPs 10A, 10B is shown. The redundant STPs 10A, 10B have only individual A-Link link sets 22, 24, 26 or 28 for connecting the STPs 102A, 102B to the respective PSTN switches 14, 16, 18, and 20. It is noted that the trunk groups 30, 32, and 34 containing the actual bearer channels between the PSTN switches 14, 16, and 18 and the central PSTN switch 20. Because the STPs 10A, 10B interconnect the signaling between the PSTN switches 14, 16, 18 and the central PSTN switch 20, it is important to the telecommunications service provider, and invariably to its customers, that the STPs 10A, 10B must not fail. Due to network redundancy in that STPs are generally provided in redundant pairs, 10A, 10B, one STP can take over the processing for a failed mate.
Disadvantageously, however, the PSTN switches 14, 16, 18, 20 are dependent on their respective single common-channel signaling interface link sets 22, 24, 26, 28 for their total SS7 network communication capabilities. If a hardware or software failure exists, or a generic software update is required, it is almost certain that a signaling outage will occur at a telephonic switch. Once this happens the switch becomes isolated and telephonic communication is lost at all telephonic units associated with the switch during the isolation period. Signaling communication at a switch following SS7 protocol is dependent on and thus, is limited by its lone signaling interface which unfortunately leads to potential switch isolation upon operational failures, upgrades to the system, or upon field events.
An additional disadvantage of A-Link link sets 22, 24, 26 being concentrated at the STPs 10A, 10B, into a single A-Link link set 28, is the difficulty in scaling up the signaling interface and controller functions of the PSTN switches 14, 16, and 18. An increase in capacity of PSTN switches 14, 16, and 18 can create signaling demands that exceeds the capacity of the A-Link link set 28 as calls are routed to the central PSTN switch 20 from the PSTN switches 14, 16, and 18. Moreover, as the call capacity increases, the impact of the A-Link link set 28 or the link controller at switch 20 failing is increased.
In order to address the capacity and fault issues, the telecommunication network can be redesigned and additional link sets and STPs can be added to the network. This approach is undesirable because of the increased cost to the telecommunication service provided. Every PSTN switch and STP added to the network requires an environmental controlled room, electrical power, additional support and maintenance investment by the telecommunication service provider thereby increasing the overall cost. Therefore, there is a need in the art to prevent disruption of telecommunication signaling while being able increase the size and capacity of PSTN switches for the telecommunication network in a cost efficient manner.
The problems noted above are solved in accordance with the invention and a technical advance is achieved in the art, by providing associated common-channel signaling link sets between the PSTN switches and a central PSTN switch. An alternate signaling path may also be provided via a STP that concentrates the A-Link link sets from the PSTN switches into an A-Link link set connected to the central PSTN switch. The associated common-channel signaling links between the PSTN switches and the central PSTN switch allow the PSTN switches to increase capacity without overloading the A-link link set and requiring increased capacity at the STP. The reliability of the telecommunication network is also increased by distributing the signaling in the network among the PSTN switches, rather than routing the signaling between the PSTN switches and the central PSTN switch through the STP and a lone A-Link link set to the central PSTN switch.
The reliability of the central PSTN switch is further increased with independent message signal interfaces. The independent message signal interfaces are individual independent interfaces that terminate an individual SS7 link set at the central PSTN switch. Moreover, the independent message signal interfaces appear as one to the SS7 network but are actually totally independent elements. If an independent message signal interface fails, the other link sets terminating at the central switch are not affected and continue to function. The signaling information from the failed link set is rerouted over an A-Link link set via the STP, where it can be processed by the central switch element so signaling can continue uninterrupted. Additionally, upon a return to normal operation of the failed link set, the signaling information is transferred back to the associated common-channel signaling link set between the PSTN switch and the central PSTN switch.